1. Field of the Invention
The present invention relates to an improved method for preparing 2,6-naphthalenedicarboxylic acid. In particular, an improvement is provided in the method for preparing 2,6-naphthalene dicarboxylic acid, which comprises the steps of; hydrolyzing di-lower alkyl 2,6-naphthalene dicarboxylate under the presence of a basic compound to give a solution of the di-basic compound salt of 2,6-naphthalene dicarboxylic acid, and aciding out the solution to give 2,6-naphthalene dicarboxylic acid.
2. Art Related
2,6-Naphthalenedicarboxylic acid (hereinafter, abbreviated as 2,6-NDA) is useful as a monomer for manufacturing a variety of polymers such as polyethylene naphthalates, liquid crystalline polyesters or polyamides. Di-lower alkyl 2,6-naphthalenedicarboxylate (hereinafter, abbreviated as 2,6-NDC) is also useful as a monomer for manufacturing a variety of polymers. Among 2,6-NDCs, dimethyl 2,6-naphthalenedicarboxylate has preferable properties such as melting point and the like, is widely employed for the preparation of high performance polymeric materials and is considered to be most important 2,6-NDC.
A conventional method for preparing 2,6-NDA includes the step of oxidizing naphthalene derivative having alkyl and/or acyl groups at their 2 and 6 positions with molecular oxygen in the presence of a heavy metal catalyst such as cobalt or manganese. Thus obtained crude 2,6-NDA, however, comprises impurities such as aldehyde intermediates and oxidized polymer, and therefore, must be purified before being employed in manufacturing polymers. Various methods for purifying the crude 2,6-NDA have been proposed.
Among the known methods for purifying crude 2,6-NDA, a method comprising the steps of estrifying the crude 2,6-NDA with a lower alcohol such as methanol to give crude 2,6-NDC, purifying the crude 2,6-NDC by means of distillation or re-crystallization, and hydrolyzing the ester group of the purified 2,6-NDC to give purified 2,6-NDA. In said method, the step of hydrolyzing ester group is proposed to be carried out with acid catalyst or base catalyst, or by means of water under a specific condition.
In a known method for hydrolyzing ester group with acid catalyst, the ester group of 2,6-NDC is hydrolyzed in the presence of acid catalyst and an aliphatic carboxylic acid to give highly pure 2,6-NDA (Japanese Patent Application Laid Open No. 6-256256, the contents of which is herein incorporated by reference). However, this method has some problems such as long processing time and production of aliphatic carboxylate esters during the ester hydrolyzing step.
A process for preparing purified 2,6-NDA which comprises hydrolyzing a 2,6-NDC with water at a reaction temperature of at least about 450° F. or 232° C. under liquid phase condition, the amount of water present being sufficient to solubilize, at the reaction temperature, at least about 10% of the 2,6-NDA formed is proposed (U.S. Pat. No. 5,563,294, the contents of which is herein incorporated by reference). Because of the high temperature as high as 232° C. and high pressure required due to the high temperature, this method is not suitable for industrial use.
In a known method for hydrolyzing the ester group of 2,6-NDC with a basic catalyst, the ester group is hydrolyzed in water or a mixed solvent consisting of water and water miscible organic solvent with the basic catalyst, and the reaction mixture is acid precipitated to harvest the 2,6-NDA (Japanese Patent Application Laid Open No. 03-240750, the contents of which is herein incorporated by reference). However, due to the very low solubility of 2,6-NDC to water or the mixed solvent, this method has problems that the 2,6-NDC cannot be hydrolyzed completely or the hydrolyzing step takes quite a long time.